The present invention relates in general to a sensor and more particularly to a measuring cell for determining oxygen concentration in a gas mixture flowing through a tube.
In connection with the problem of reducing air pollution resulting from the automobile internal combustion engine, it is well known that if the air to fuel ratio of the intake charge to the engine is maintained at or near stoichiometric condition during most modes of operation, the exhaust gases will contain less harmful components, i.e., hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO.sub.x). For controlling the air to fuel ratio of the intake charge at the stoichiometric condition, a so-called closed loop system having an oxygen sensor placed in communication with the exhaust gases issued from the engine has been widely used. The oxygen sensor is constructed to generate an electrical signal responsive to the oxygen content of the exhaust gases. The electrical signal in turn is received in a control means connected to the engine for regulating or varying the charactor of the intake air-fuel charge so as to maintain the charge at the stoichiometric condition.
Hitherto, stabilized zirconium oxide (ZrO.sub.2) has been widely employed as a main element of the oxygen sensor. As is well known, the stabilized zirconium oxide (ZrO.sub.2) exhibits conductivity by means of oxygen ions which transfer therethrough. In reality, if some gas mixture whose partial oxygen pressure or absolute oxygen pressure must be measured is present on one side of a partition member made of the zirconium oxide, and simultaneously, a reference gas having a known partial oxygen pressure is present on the other side, a considerable voltage difference (E) is generated by the movement of the oxygen ions between the one and the other sides of the partition member. The magnitude of the voltage difference (E) is generally estimated by the next Nernst equation; ##EQU1## where: R . . . gas constant
T . . . absolute temperature PA1 F . . . Faraday constant PA1 P.sub.1 . . . partial oxygen pressure of the reference gas PA1 P.sub.2 . . . partial oxygen pressure of the unknown gas mixture
With this equation, it will be appreciated that the partial oxygen pressure of the unknown gas mixture and accordingly the oxygen concentration of the same are calculated by measuring the voltage difference (E). By the way, it was revealed that the oxygen concentration of the exhaust gases issued from the engine is critically dependent upon the air to fuel ratio of the intake charge to the engine.
Apart from this, it has been observed that the zirconium oxide oxygen sensor does not generate sufficient voltage difference at low temperature. In fact a sufficient voltage difference for measuring the oxygen concentration cannot be expected at a temperature below about 350.degree. C. Therefore, when equipped in the exhaust tube of the engine, the zirconium oxide oxygen sensor must be located in a position where a highest possible temperature of the exhaust gases from the engine exists. FIG. 2 shows the distribution of temperature in the exhaust tube in a case where the displacement of the associated engine is 2000 cc and the inside diameter of the exhaust tube is 45 mm. The curves a and b represent the respective temperature distribution in two cases wherein the engine speeds are 1200 rpm and 800 rpm, respectively. From this Figure, it will be noted that in the exhaust tube, a temperature difference ranging from about 150.degree. C to about 200.degree. C will appear in each engine operation mode. Furthermore, the temperature gradient is maximum in a region between the inner surface of the exhaust tube and a portion about 10 mm away from the inner surface of the tube. These phenomena similarly occur also in other cases wherein the displacement of the engine changes from about 1000 cc to about 4000 cc and the inside diameter of the exhaust tube changes from about 40 to 60 mm. From this description, it will be appreciated that the zirconium oxide oxygen sensor should be arranged in such a manner that the sensitive part thereof is located in a position at least 10 mm away from the inner surface of the exhaust tube.